Optical rotary encoders are widely used for precisely detecting a position of a moving industrial machine such as a numerically controlled apparatus (NC). The encoder is often installed in poor environmental locations, e.g., areas of extreme temperatures and considerable dust, etc. In addition, the encoder is often separated by several tens of meters from a control apparatus of the NC using a position detection signal from the encoder as a control signal. In a conventional encoder, light emitted from a light emitting diode is received by a light receiving element such as a photodiode through a slit formed in a rotating plate, and a signal from the light receiving element is generated as a rotational position signal through signal processing. For this reason, the encoder installed at the detecting site incorporates a light emitting diode, a photodiode, and a signal processing circuit. In the conventional encoder of this type, the signal processing circuit generally comprises a semiconductor element, and when the ambient temperature of the encoder becomes high, the signal processing circuit may not operate normally.
Since an output from the signal processing circuit is a low level signal of, e.g., about 5 VDC, when the encoder is separated from the control apparatus by a long distance, the output from the signal processing circuit may be influenced by electrical noise from a power cable. In order to overcome this drawback, twisted pair shield cables are used, and the encoder is located separately from the power cables. However, this results in increased cost and inconvenience.
Encoders are divided into incremental and absolute types. In the former encoder, light shielding portions and light transmission portions (slits) are provided along a periphery of a rotating plate which is fixed to a rotating shaft for detecting a movement of an industrial machine, as described above. A light emitting diode emits light from one side of the rotating plate, and a photodiode is provided at the other side of the rotating plate so as to receive the light emitted from the light emitting diode. When the rotating plate is rotated in a normal direction, the encoder generates an incremental signal, and otherwise, generates a decremental signal. In the latter encoder, a plurality of light transmission channel arrays each having a predetermined pattern along a radial direction of a rotating plate are provided along the periphery of the rotating plate, and light emitting diodes are provided at one side of the rotating plate, so that light emitted from the light emitting diodes is received by a photodiode array corresponding to the light transmission channels. Thus, a position of the rotating shaft is uniquely generated by a combination of the light reception signals of the photodiodes, i.e., in an absolute mode.
In the incremental type encoder, since the rotating plate has a simple structure, the signal processing circuit can be simplified. However, since the encoder is powered by the control apparatus, a position signal from the rotation plate is lost upon cutting-off or failure of a control power source. Therefore, when power is turned on again, unless a count is restarted from a standard position, a position of the rotating plate cannot be detected. On the other hand, the absolute type encoder has an advantage that a position of the rotating plate can be quickly detected by a light receiving pattern of the photodiode array even when the power is turned on again after power is cut off. However, since the rotating plate has a complex structure and many light receiving elements are provided, the signal processing circuit becomes complicated.
In either encoder, a light emitting diode is used as a light emitting element, and a photodiode is used as a light receiving element. However, due to strong directivity, the light emitting diode must be three-dimensionally adjusted upon observation by a synchronous scope or the like in order to obtain good light receiving characteristics at the photodiode, thereby complicating assembly and adjustment. Similarly, adjustment and inspection such as microscopic inspection are needed for detecting whether or not the rotating plate is eccentric about its central axis and for aligning a phase of the stationary slit.
Furthermore, in either type of conventional encoder, a rotational position of the rotating plate is detected only during one revolution, and a rotation of the rotating plate itself is not held as an output from the encoder For this reason, a calculation for obtaining an overall moving distance of the industrial machine must be performed at the control apparatus side of the NC, and a desired position signal cannot be obtained simply by the output signal from the encoder. Therefore, problems occur from the viewpoint of total signal processing.
Also, the conventional encoder has a relatively large power, consumption.